Tamoxifen remains the endocrine therapy of choice in the treatment of all stages of hormone-dependent breast cancer. In addition, large-scale clinical trials are in progress to determine the potential of tamoxifen to act as a chemopreventive agent in women considered at high risk for developing breast cancer. However, several studies have raised concern over the safety of chronic treatment with this drug. Alternate antiestrogens including droloxifene, toremifene, and idoxifene, may not be genotoxic probably because of different routes of metabolism which could lead to a decrease in amount and/or type of ultimate carcinogen(s). The central hypothesis of this project is that the formation of reactive intermediates is an important mechanism of carcinogenesis and/or cytotoxicity for certain antiestrogens. For example, tamoxifen can be metabolized to at least three electrophilic metabolites of very different reactivity: quinone methides, carbocations, and o-quinones. The following specific aims are proposed: 1. Role of quinone methides, carbocations, and/or o-quinones in the carcinogenic and cytotoxic effects of antiestrogens. The carcinogenic potential of the proximate carcinogens from tamoxifen, droloxifene, toremifene, and idoxifene will be studied in C3H1 OT1/2 cells and their tumor promoting ability examined in JB6 cells. The biochemical effects of the antiestrogens and their metabolites will be investigated in human breast and endometrial cancer cell lines. 2. Investigate the effect of reactive metabolite structure on electrophilic and/or redox reactivity. The substituent effects on the electrophilicity/redox ability of the antiestrogen reactive intermediates will be investigated by measuring their ability to alkylate/oxidize DNA. Redox active metabolites will be tested by monitoring changes in reduced cofactors and by determining the formation of reactive oxygen species. 3. Determine if the antagonist/agonist activity of antiestrogen metabolites correlates with the extent of DNA damage in estrogen receptor positive cell lines. The Ishikawa cell system will be used to determine the estrogenic antiestrogenic and/or toxic effects of the proximate hydroxylated metabolites and the ultimate reactive intermediates. Cellular DNA from estrogen receptor positive and negative cells lines will be isolated after treatment with the test compound. The DNA will be hydrolyzed to deoxynucleosides (identified from Aim 2) and examined for covalent adducts and oxidative damage. These studies will greatly assist in the design of estrogen antagonists that maintain beneficial properties without generating genotoxic metabolites.